Many same extruded windows
As Lukasz-40sth said, you could use the Array modifier:


Create a plane, subdivide it.
Extrude your window.
Use 2 Array modifiers to repeat your object horizontally and vertically.
Work your window as much as you want until you're satisfied.
You can either keep or apply your modifiers.
If you want to apply, enable the Merge option of the modifier so that the adjacent vertices of your new object merge. If you didn't enable it, go in Edit mode, select all and Remove Doubles with W.